Solid matter takes on new behavior

Investigating how solid matter behaves at the enormous pressures found deep inside giant planets is a major experimental challenge. To address this challenge, researchers and collaborators at Lawrence Livermore National Laboratory (LLNL) delved deep into understanding these extreme loads.

The work has just been published in natural physics with LLNL scholar Martin Gorman as lead author.

“Our results represent a significant experimental advance; we were able to study the structural behavior of magnesium (Mg) at extreme pressures – more than three times higher than at the Earth’s core – that were previously only theoretically accessible,” said Gorman. “Our observations confirm theoretical predictions for Mg and show how TPa pressures – 10 million times atmospheric pressure – are forcing materials to adopt fundamentally new chemical and structural behaviors.”

Gorman said modern computational methods suggest that at extreme pressures, core electrons bound to neighboring atoms begin to interact, causing the conventional rules of chemical bonding and crystal structure formation to break down.

“Perhaps the most striking theoretical prediction is the formation of high-pressure ‘electrodes’ in elemental metals, where free valence band electrons are squeezed into localized states within the voids between ions to form pseudoionic configurations,” he said. “But achieving the required pressures, often above 1 TPa, is experimentally very challenging.”

Gorman explained the work by describing the best way to arrange balls in a barrel. Conventional wisdom holds that atoms under pressure, like bullets in a barrel, should prefer to stack as efficiently as possible.

“To put the maximum number of balls in a run, they must be stacked as efficiently as possible, such as in a hexagonal or cubic close-packed pattern,” Gorman said. “But even the tightest packings are only 74% efficient and 26% is still empty space, so by including smaller, properly sized balls, more efficient packing of balls can be realized.

“Our results indicate that the valence electrons, which are normally free to move through the Mg metal, localize under immense pressure in the empty spaces between atoms, forming a nearly massless, negatively charged ion,” he said. “Now there are spheres of two different sizes – positively charged Mg ions and negatively charged localized valence electrons – which means that Mg can pack more efficiently and therefore such ‘electride’ structures become energetically favorable over close packing.”

The work detailed in the paper required six days of shooting at the National Ignition Facility (NIF) between 2017 and 2019. Members of an international collaboration traveled to the LLNL to observe the firing cycle and help with data analysis on the days following each experiment.

The state-of-the-art high-power laser experiments at the NIF, coupled with nanosecond X-ray diffraction techniques, provide the first experimental evidence – in any material – for the formation of electride structures above 1 TPa.

“We ramped elemental Mg, maintained the solid to a peak pressure of 1.32 TPa (more than three times the pressure at the center of the Earth), and watched Mg transform into four new crystal structures,” Gorman said. “The structures formed are open and have inefficient atomic packing, contradicting our traditional understanding that spherical atoms in crystals should pack more efficiently with increasing compression.”

However, it is precisely this atomic packing inefficiency that stabilizes these open structures at extreme pressures, since the empty space is needed to better accommodate localized valence electrons. Direct observation of open structures in Mg is the first experimental evidence of how valence-nucleus and core-nucleus electron interactions can affect material structures at TPa pressures. The transformation observed between 0.96 and 1.32 TPa is the highest-pressure structural phase transition observed in any material to date, and the first at TPa pressures, according to the researchers.

Gorman said that these kinds of experiments can only be done at the NIF at the moment and open the door to new areas of research.